High performance knee prostheses

ABSTRACT

Knee prostheses that more faithfully replicate the structure and function of the human knee joint in order to provide, among other benefits: greater flexion of the knee in a more natural way by promoting or accommodating internal tibial rotation, replication of the natural screw home mechanism, and controlled articulation of the tibia and femur respective to each other in a natural way. Such prostheses may include an insert component disposed between a femoral component and a tibial component, the insert component featuring, among other things, a reversely contoured posterolateral bearing surface that helps impart internal rotation to the tibia as the knee flexes. Other surfaces can be shaped using iterative automated techniques that allow testing and iterative design taking into account a manageable set of major forces acting on the knee during normal functioning, together with information that is known about natural knee joint kinetics and kinematics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/642,242 filed Mar. 9, 2015, which is acontinuation application of U.S. patent application Ser. No. 13/659,309filed Oct. 24, 2012, which is a continuation application of U.S. patentapplication Ser. No. 12/582,300 filed Oct. 20, 2009, now U.S. Pat. No.8,398,715, which is a continuation application of U.S. patentapplication Ser. No. 12/023,112 filed Jan. 31, 2008, now U.S. Pat. No.7,922,771, which is a continuation application of U.S. patentapplication Ser. No. 10/743,885 filed Dec. 22, 2003, now U.S. Pat. No.7,326,252, which claims priority from U.S. Provisional Application No.60/435,426 entitled “Knee Prosthesis Having Improved Stability andRotational Control” filed Dec. 20, 2002, the entire contents of all ofwhich are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention relates generally to knee prostheses and, morespecifically, to knee prostheses which more closely emulate the anatomyand function of the knee and thereby feature range of flexion, rotationof the tibia relative to the femur, the screw home mechanism, and otherstructural and functional characteristics of the actual knee joint.

2. General Background of the Invention

Disease and trauma affecting the articular surfaces of the knee jointare commonly treated by surgically replacing the ends of the femur andtibia with prosthetic femoral and tibial implants, and, in some cases,replacing the patella with a patella component. Such surgeries aresometimes referred to as total knee replacement (TKR). In TKR surgery, asurgeon typically affixes two prosthetic components to the patient'sbone structure; a first to the patient's femur and a second to thepatient's tibia. These components are typically known as the femoralcomponent and the tibial component respectively.

The femoral component is placed on a patient's distal femur afterappropriate resection of the femur. The femoral component is usuallymetallic, having a highly polished outer condylar articulating surface,which is commonly J-shaped.

A common type of tibial component uses a tray or plateau that generallyconforms to the patient's resected proximal tibia. The tibial componentalso usually includes a stem that extends at an angle to the plateau inorder to extend into a surgically formed opening in the patient'sintramedullary canal. The tibial component and tibial stem are bothusually metallic.

A plastic or polymeric (often ultra-high molecular weight polyethylene)insert or bearing fits between the tray of the tibial component and thefemoral component. This insert provides a surface against which thefemoral component condylar portion articulates, i.e., moves in grossmotion corresponding generally to the motion of the femur relative tothe tibia.

Modern TKR's are tricompartmental designs; they replace three separatearticulating surfaces within the knee joint: the patello-femoralcompartment and the lateral and medial inferior tibio-femoralcompartments. Most currently available TKR's are designed to articulatefrom a position of slight hyperextension to approximately 115 to 130°flexion. A tricompartmental design can meet the needs of most TKRpatients even though the healthy human knee is capable of a range ofmotion (ROM) approaching 170° However, there are some TKR patients whohave a particular need to obtain high flexion in the knee joint. Formany, a TKR that permits patients to achieve a ROM in excess of 130° isdesirable to allow deep kneeling, squatting and sitting on the floorwith the legs tucked underneath.

Additionally, a common complaint of TKR patients is that the replacedknee does not does function like a normal knee or “feel normal.” Thereplaced knee does not achieve normal knee kinematics or motion andgenerally has a more limited ROM than a normal knee. Currently availabledesigns produce kinematics different than the normal knee during gait,due to the complex nature of the knee joint and the motion of the femurand tibia relative to one another during flexion and extension. Forexample, it is known that, in addition to rotating about a generallyhorizontal axis during flexion and extension, the tibia also rotatesabout its longitudinal axis. Such longitudinal rotation is typicallyreferred to as either external or internal rotation, depending onwhether reference is being made to the femur or tibia respectively.

Very few currently available designs allow this longitudinal rotation.One known method to allow rotation is a mobile-bearing knee prosthesis.In mobile-bearing knee prostheses, the insert has increased contact withthe condyles of the femoral component and rotates on top of the tibialcomponent. However, mobile-bearing knee prostheses are less forgiving ofsoft tissue imbalance, increasing the incidence of bearing spin-out anddislocation. Another concern is that the mobile-bearing prosthesescreate an additional interface and underside wear may occur.

Constructing a total knee prosthesis which replicates the kinematics ofa natural knee has been an on-going challenge in the orthopaedic field.Several attempts have been made and are well known in the prior art,including those shown in U.S. Pat. Nos. 6,264,697 and 6,325,828.Conventional designs such as these, however, leave room for improvementin simulating the structure and operation of actual knee joints, in atleast the aspects of range of motion, internal rotation of the tibiarelative to the femur as the knee flexes, and rotation of the tibiarelative to the femur in overextension in order to allow the knee to bestabilized more efficiently.

SUMMARY

Devices according to aspects of the invention achieve more faithfulreplication of the structure and function of the actual knee joint by,among other things, adoption and use of structure and shaping of atleast the polymeric insert and the femoral component to cause thesecomponents to cooperate with each other in new and unconventional ways(at least in the art of prosthetics) at various stages throughout therange of knee motion.

According to certain aspects and embodiments of the invention, there isprovided a knee prosthesis in which the insert features a lateralposterior surface which slopes in a distal direction (as compared to thecorresponding medial posterior surface) as it continues toward theposterior aspect of the insert, in order to cooperate with the lateralcondyle of the femoral component to impart internal rotation to thetibia as the knee flexes between substantially 0 and substantially 130degrees of flexion, to allow the prosthesis to induce or allow tibialinternal rotation in a controllable manner as a function of flexion, toreduce the forces of any femoral component cam acting upon a post orother raised portion of the insert, or any combinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features agreater thickness in certain lateral portions to increase durability,accommodate a more anatomic femoral component which features a lateralcondyle smaller in some dimensions than its medial condyle, to impart ajoint line more accurately replicating natural physiology, or anycombinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features moreanatomic sulcus placement in order improve operation of the prosthesisby more anatomically applying forces imposed on the prosthesis byquadriceps and the patellar tendon, allow the prosthesis to replicatenatural anatomy more effectively, or any combinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features alateral surface that is curved or “swept” in plan, in order to allow thelateral condyle to track in arcuate fashion on the bearing surface atcertain ranges of knee flexion and rotation, to assist in facilitatingthe screw home mechanism, or combinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features a postor other raised portion whose anterior surface is shaped to serveeffectively as an anterior cruciate ligament when engaged with a camduring ranges of flexion such as after heel strike upon actuation of thequadriceps.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features a postor other raised portion whose posterior surface is shaped to assistinternal rotation of the tibia relative to the femur as the knee flexes,such as starting at angles such as in a range of substantially 50 ormore degrees, to help ensure that post/cam forces are directed netanteriorly, or a combination of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis in which the insert features roundedor chamfered peripheral edges to help reduce wear on surrounding tissueand/or for other purposes.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis with any desired combination orpermutation of any of the foregoing features, properties or results.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component thatincludes a lateral condyle that is in some distal and posterior aspectssmaller than corresponding dimensions of the medial condyle, in order tosimulate more closely natural physiology, allow adequate insertthickness under the lateral condyle so that, for instance, theposteriolateral surface of the insert can feature convexity or slope,assist internal rotation of the tibia relative to the femur as the kneeflexes from substantially 0 degrees to substantially 130 degrees, or anycombinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component thatincludes a lateral condyle with anterior surfaces more pronounced thancorresponding anterior surfaces on the medial condyle, in order toreplicate more closely natural anatomic structures in retaining thepatella in lower ranges of flexion, cause the patella or substitutestructure to track more physiologically at such ranges of motion, causethe quadriceps more physiologically to apply force to the prostheticcomponents and tibia in lower ranges of flexion, or any combinations ofthese.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component thatincludes a cam that cooperates with a post or other raised portion onthe insert to assist internal rotation on the tibia, ensure thatcam/post forces are directed net anteriorly or a combination of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component thatincludes an anterior cam which cooperates with a post or other raisedportion on the insert to simulate action of the anterior cruciateligament at lower ranges of flexion.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component and aninsert in which during operation in situ, the femoral component issituated more anteriorly on the insert at low angles of flexion than inconventional knee prostheses, in order to reduce the forces on the postof the insert, to resemble more closely actual operation and kinematicsof the knee, or a combination of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component and aninsert which during operation in situ reduces paradoxical motion andactual cam to post contact, and when there is contact, reduces impact ofcontact and force of contact, between the femoral component cam and theinsert post or other raised portion during desired ranges of motion.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis including a femoral component whichfeatures a backdrafted anterior slope of the interior surfaces of theposterior condylar portions, in order to allow the distal portion of thefemur to be resected so that the anterior cut and the posterior cut arenot parallel, such that the distal extremity of the femur is physicallygreater in anterior-posterior dimension than portions more proximal,whereby the distal extremity of the femur can be physically captured bythe interior surfaces of the femoral component.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis which helps impart internal rotationto the tibia as the knee flexes from substantially 0 degrees of flexionto substantially 130 degrees of flexion, such that the tibia issubstantially fully internally rotated to an angle of at leastapproximately 8 degrees in order to allow such flexion to occur in morephysiological fashion, to reduce the possibility that the quadricepswill pull the patella undesirably relative to the knee in a lateraldirection (lateral subluxation), to allow the patella or its replacementto track the trochlear groove, or any combinations of these.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis which helps impart internal rotationof the tibia as the knee flexes between substantially zero degrees andsubstantially 130 degrees, to at least substantially 8 degrees ofinternal rotation of the tibia relative to the femur at flexion anglesgreater than 130 degrees.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis which imparts internal rotation ofthe tibia relative to the femur as the knee flexes from substantially 0degrees to substantially 130 degrees of flexion, so that the tibia issubstantially fully internally rotated relative to the femur to an angleof at least substantially 8 degrees at a flexion angle of substantially130 degrees, such flexion and internal rotation of the tibia beingfacilitated at least in part by a twisting moment created by contact ofthe condyles of the femoral component on the insert.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis which imparts internal rotation ofthe tibia relative to the femur as the knee flexes from substantially 0degrees to substantially 130 degrees of flexion, so that the tibia issubstantially fully internally rotated relative to the femur to an angleof at least substantially 8 degrees at a flexion angle of substantially130 degrees, such flexion and internal rotation of the tibia beingfacilitated at least in part by a twisting moment created by contactbetween the post or other raised portion of the insert and at least onecam of the femoral component.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis whose structure facilitates the screwhome mechanism.

According to certain aspects and embodiments of the invention, there isfurther provided a knee prosthesis which allows flexion at flexionangles greater than 130 degrees while allowing internal rotation of thetibia relative to the femur as the knee flexes from substantially 0degrees to substantially 130 degrees, without the need for a mobilebearing design or to allow the insert to swivel or rotate relative tothe tibial component.

According to certain aspects and embodiments of the invention, there areprovided methods of designing knee prosthetic components usingsimulation of a femoral, patella and insert structure, physiologicaldata regarding structure and function of natural knees, and applying atleast six force vectors to the structure throughout a desired range ofmotion to effectively and efficiently simulate forces applied to thetibia in the body: force applied by the patella ligament, groundreaction force, relative force applied by the lateral condyle on theinsert, relative force applied by the medial condyle on the insert,force applied by the hamstring muscles, and relative force applied bythe cam surfaces of the femoral component on the post or other raisedportion of the insert.

According to certain aspects and embodiments of the invention, there areprovided methods of designing knee prosthetic components usingsimulation of a femoral and insert structure and applying to thestructure throughout a desired range of motion, force vectors thatrepresent relatively greater forces applied by some ligaments, tendonsand muscles than others, such as the relatively great forces applied bythe quadriceps when they actuate and by the hamstrings when theyactuate.

According to certain aspects and embodiments of the invention, there areprovided methods of designing knee prosthetic components usingsimulation of a femoral and insert structure and applying to thestructure a desired set of forces, evaluating the performance of thestructure, modifying the structure as simulated in the computer, andrepeating the process until a desired design is reached.

According to additional aspects and embodiments of the invention, thereis provided a knee prosthesis comprising: a femoral component adapted tofit on a distal end of a femur, the femoral component including alateral condylar structure and a medial condylar structure, the geometryof the lateral condylar structure being different from the geometry ofthe medial condylar structure; and an accommodation structure includinga lateral proximal surface adapted to cooperate with the lateralcondylar structure of the femoral component, and a medial proximalsurface adapted to cooperate with the medial condylar structure of thefemoral component, the geometry of the lateral proximal surface and themedial proximal surface being different from each other, to assist inimparting internal rotation on the tibia relative to the femoralcomponent as the knee flexes from substantially zero degrees of flexionto substantially 130 degrees of flexion.

According to additional aspects and embodiments of the invention, thereis provided a knee prosthesis comprising a femoral component adapted tofit on a distal end of a femur, the femoral component including: ananterior portion which includes an interior surface adapted to interfacewith the femur; a lateral condylar structure which includes a posteriorsection which in tum includes an interior surface adapted to interfacewith the femur; and a medial condylar structure which includes aposterior section which in turn includes an interior surface adapted tointerface with the femur; wherein the interior surfaces are adapted tophysically capture at least a portion of the femur in the femoralcomponent relative to a distal translation substantially parallel to theanatomic axis of the femur; and wherein all interior surfaces of thefemoral component are adapted to allow the femoral component to clearresected portions of the femur physically as the femoral component isrotated onto the femur about its posterior portions during installation.

Certain embodiments and aspects of the invention also provide othercharacteristics and benefits, and other objects, features and advantagesof various embodiments and aspects of the invention will be apparent inthe other parts of this document.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of a left knee prosthesis according toan embodiment of the invention.

FIGS. 1B-1C show an exploded front perspective view of a femoralcomponent and an insert of a left knee prosthesis according to anembodiment of the invention.

FIG. 2 shows an exploded back perspective view of a femoral componentand an insert of a left knee prosthesis according to an embodiment ofthe invention.

FIG. 3 shows an exploded front perspective view of a femoral componentand an insert of a left knee prosthesis according to an embodiment ofthe invention.

FIG. 4 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the left kneeat full extension.

FIG. 5 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the knee at 30°flexion.

FIG. 6 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the knee at 60°flexion.

FIG. 7 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the knee at 90°flexion.

FIG. 8 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the knee at120° flexion.

FIG. 9 is a side view of portions of a left knee prosthesis according toan embodiment of the invention showing the kinematics of the knee at130° flexion.

FIG. 10 is a side view of portions of a left knee prosthesis accordingto an embodiment of the invention showing the kinematics of the knee at140° flexion.

FIG. 11 is a side view of portions of a left knee prosthesis accordingto an embodiment of the invention showing the kinematics of the knee at150° flexion.

FIG. 12 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at full extension.

FIG. 13 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 30° flexion.

FIG. 14 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 60° flexion.

FIG. 15 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 90° flexion.

FIG. 16 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 120° flexion.

FIG. 17 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 130° flexion.

FIG. 18 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 140° flexion.

FIG. 19 is a top plan view of portions of a left knee prosthesisaccording to an embodiment of the invention showing the kinematics ofthe knee at 150° flexion.

FIG. 20 shows a front plan view of a left knee prosthesis according toan embodiment of the invention.

FIG. 21 shows certain aspects of a femoral component of a kneeprosthesis according to an embodiment of the invention.

FIG. 22 shows certain aspects of a cam of a femoral component of a kneeprosthesis according to an embodiment of the invention.

FIG. 23 shows certain aspects of a proximal surface of an insert of aknee prosthesis according to an embodiment of the invention.

FIG. 24 is a cross sectional view showing certain aspects of a lateralbearing surface of a knee prosthesis according to an embodiment of theinvention.

DETAILED DESCRIPTION

Various embodiments of the invention provide improved knee prosthesesfor replacing at least a portion of a knee joint between the distal endof a femur and the proximal end of a tibia.

While not wishing to be bound by any particular theory, the inventorshave discovered that knee prostheses which more faithfully and closelyreplicated the function, anatomy and physiology of the normal human kneewould yield a number of advantages. Among other things, such prostheseswould provide an increased range of motion and would function morenormally particularly in extension, deep flexion and during normal gait.They would take into account the forces imposed on the knee byquadriceps and hamstrings actuation, forces which great in magnitude butnot fully considered in conventional knee prosthesis design. Kneeprostheses according to various aspects of the invention recognize thatduring movement of the knee, particularly during flexion, the positionand orientation (kinematics) of the bones of the knee are a result ofachieving equilibrium of the forces that cause motion of the knee(kinetics). Additionally, the shape of the articular surfaces (anatomy)acting in combination with forces imposed by various muscles, ligamentsand tendons, determines the direction of the large contact forces.Therefore, aspects of the invention take into account that anatomyinfluences kinetics and kinetics determine kinematics.

Conventional knee prostheses have been developed without recognition ofthe full range of kinetics of active knee movement. Many are primarilyconcerned with achieving greater flexion. However, in addition toflexion and extension, motion of the knee is both rotational andtranslational. The femoral condyles both roll and glide as theyarticulate with respect to the tibial plateaus. As the knee moves fromfull extension into flexion the axis of rotation between the femur andthe tibia moves posteriorly relative to both the femur and the tibia.Additionally, in the normal human knee, internal rotation of the tibiarelative to the femur occurs as the knee flexes between full extensionand approximately 130° of flexion. Knee prostheses according to variousaspects of the invention provide various surfaces on at least thefemoral component and the insert which promote such greater flexion, thescrew home mechanism, internal rotation of the tibia relative to thefemur as the knee flexes, and other characteristics of the natural knee.

According to some aspects of the invention, the design of kneeprosthesis components is conducted using a process which (1) testsvarious performance aspects of a proposed design using computersimulation of the design and various forces imposed upon it, (2) allowsanalysis of the test results for development of improvements to theproposed design; (3) uses test results to change the proposed design(either manually or automatically), (4) tests various performanceaspects of the modified design using computer simulation of the designand various forces imposed upon it, and (5) repeats these tasks in aniterative fashion until the performance testing shows an iterativelymodified design to feature acceptable performance characteristics. It isalso significant that in such performance testing, the performance ofthe proposed design is tested using forces that occur at various pointsin various activities, so that the performance testing is dynamic acrossextended ranges of motion and takes into account considerable forcesplaced on the design by actuation of the quadriceps and hamstringmuscles, for example, and the consequent kinetic and kinematic effectsof such forces.

A preferred embodiment of a knee prosthesis according to the inventionis shown in FIGS. 1A-1E and 2-4, and identified by the numeral 100. Theknee prosthesis 100 shown in these figures is designed to replace atleast a portion of a left knee joint between the distal end of a femurand the proximal end of a tibia. A mirror image (not shown) of kneeprosthesis 100 will replace at least a portion of a right knee betweenthe distal end of a femur and the proximal end of a tibia.

The knee prosthesis 100 includes a femoral component 200 for mounting toa distal end of a femur, a tibial component 300 for mounting to aproximal end of a tibia, and an insert 400.

Embodiments of the femoral component 200 preferably include a medialcondylar section 202, a lateral condylar section 204 and a trochleargroove 206 joining the anterior portions 214, 216 of the medial andlateral condylar sections 202, 204 together. The medial and lateralcondylar sections 202, 204 are disposed apart from one another to forman intercondylar recess or notch 208. Each condylar section 202,204 hasan outer surface 210, 212 for engaging a tibial component 300 or insert400 as will become apparent. The outer surfaces 210, 212 of eachcondylar section 202, 204 preferably have distal portion 218, 220 forengaging a portion of the tibial component 300 or insert 400 when theknee joint is extended and partially flexed, and posterior portions 222,224 for engaging a portion of the tibial component 300 or insert 400when the knee joint is flexed at angles of substantially 90° or greater.

Embodiments of a femoral component 200 according certain aspects of thisparticular nonlimiting embodiment of the invention also replicate thephysiological joint line 227 of a normal knee as shown in FIG. 20. Thephysiological joint line 227 may be considered to be a line extendingbetween the distal most portions of each condyle at a knee flexion angleof zero degrees. This physiological joint line is oriented at an angleof approximately 93 degrees from the mechanical axis of the leg (whichcould also be considered to be 87 degrees from the mechanical axis ofthe leg depending on perspective), or approximately 3 degrees fromhorizontal as shown in FIG. 20. The joint line established by prosthesesaccording to certain embodiments and aspects of the invention preferablyreplicate this physiological joint line 227 as shown in that drawing.

Embodiments of the femoral component 200 preferably have a thicknessapproximately matching the bone resection necessary for the total kneereplacement.

Embodiments of the femoral component 200 also preferably have a lateralcondylar section 204 that is different in geometry than the geometry ofthe medial condylar section 202. In the embodiment shown in FIG. 1, thesize of lateral condylar section 204 is smaller than the size of medialcondylar section 202 so that its outer surface distal portion 220 doesnot extend as far distally as does the outer surface distal portion 218of medial condylar section 202.

The femoral component 200 may include a rounded medial profile.According to certain embodiments, for example, it may feature a medialprofile which includes a single radius from 15-160°, and may alsoinclude a lateral profile that is less round or curved distally, with asingle radius from 10-160°.

In the normal human knee, the patella glides caudally on the femoralcondyles from full extension to full flexion. By 20 to 30° of flexion,the patella first begins to articulate with the trochlear groove. Atextreme flexion, the patella lies in the intercondylar recess. Initiallythe patella contact occurs distally and with increased flexion thecontact areas shift proximally on the patella. Patellofemoral contactforce is substantially body weight when walking, and increases tosubstantially 5 times body weight when stair climbing. These contactforces therefore impose a substantial load on the knee joint, whichprostheses according to certain embodiments and aspects specificallytake into account.

Knee prostheses according to certain embodiments and aspects of theinvention incorporate features that allow the patellar implant of theknee prostheses to move in a way similar to the normal human knee and towithstand the normal patellofemoral contact force without unnecessaryligament release. These features include various aspects of the shape ofportions of the medial condylar section 202 and the lateral condylarsection 204, to be more consistent with natural anatomical geometry. Forinstance, anterior portion 216 of lateral condylar section 204 can beconfigured to extend further anteriorly than anterior portion 214 ofmedial condylar section 202, or to be more abruptly shaped on itssurface that cooperates with the patella, so that it acts as a buttressto guide the patella at low flexion angles and in extension.

Femoral components according to certain embodiments and aspects of theinvention can also include a patella-friendly trochlear groove 206. Thetrochlear groove 206 in such embodiments is substantially S-shaped andlateralizes the patella 500. The trochlear groove 206 further allows fora smooth transition between the anterior portions 214, 216 of thecondylar sections and intercondylar notch 208. This further reduces thecontact forces on the patella 500.

Femoral components 200 according to certain embodiments and aspects ofthe invention can include flexed or backdrafted substantially planarinterior or bone interface surfaces 223 and 225 (collectively,backdrafted surface 229), on the anterior surfaces of posterior portionsof medial condyle section 222 and lateral condyle section 224.Preferably the interior surfaces 223, 225 are coplanar and are orientedso that their planes converge with a plane formed by the interiorsurface 215 on the posterior side of anterior portions 214 and 216 ofthe femoral component 200 as shown more clearly in FIG. 21. In this way,proximal portions of these posterior condylar interior surfaces 223 and225 are located closer to the plane of the interior surface 215 of theanterior portion of the femoral component 200 than are distal portionsof surfaces 223 and 225. Preferably, the convergence angle is in a rangeof between 1 and 30 degrees, and more preferably, the convergence angleis approximately 15 degrees. The backdrafted surface 229 extends thearticular surface of the femoral component 200 with minimal boneresection. Removing less bone decreases the likelihood of later femoralfracture. It also minimizes the likelihood that the femoral component200 will be forced off the end of the femur in deep flexion, since itserves to lock onto or capture the distal end of the femur in thefemoral component 200.

The femoral component 200 with the backdrafted surface 229 can beinstalled by hinging and rotating the femoral component 200 onto theresected femur about the posterior portions of the condyles of thefemur. The inventors have discovered that it is possible, by configuringall anterior surfaces of the femoral component 200 correctly, as shownin FIGS. 4-11 and 21, for example, to allow those surfaces to physicallyclear the resected bone as the femoral component is rotated onto thefemur during installation. Among other ways to accomplish thisconfiguration are: (1) to cause the interior surfaces to create ashallow interior space; and/or (2) to adjust angles and/or dimensions ofthe chamfered surfaces that connect the interior surfaces 223, 225 ofcondylar sections 202 and 204 and/or interior surface 215 of theanterior portion of the component 200 to the bottom interior surface ofthe component 200.

Interior surfaces of the component 200, including surfaces 215, 223 and225, need not be planar or substantially planar to accomplish theobjective of capturing or locking onto the femur. For instance, one ormore of them may be curved or partially curved and accomplish thisobjective by orienting one or both of the interior surfaces of thecondylar sections 202, 204 relative to the interior surface of theanterior portion of the femoral component at other than parallel.

Certain embodiments of the femoral component 200 may include an anteriorcam 230, as shown in FIGS. 4-11. As explained further below, theanterior cam 230 works with the post or other raised portion 422 of theinsert 400 to provide anterior stabilization during early gait. Theanterior cam 230 preferably includes a large radius to increase thecontact area between the anterior cam 230 and the post 422. The anteriorcam surface 230 preferably does not engage the anterior surface of thepost 422 for approximately 1-2 mm.

Certain embodiments of the femoral component 200 may include a posteriorcam 232 as shown in FIGS. 4-11, among other places as well as in acloser view in FIG. 22. Preferably, the posterior cam 232 isasymmetrical. The lateral side 238 may be larger than the medial side240, for example, as shown in FIG. 22. As explained further below, thelarger lateral side 238 provides optimal contact between the posteriorcam 232 and the post 422 during axial rotation, to assist in impartinginternal rotation to the tibia relative to the femur as the knee flexes.In general, the posterior cam 232 engages the post 422 between 50-60°flexion. The post 422 may be thickened distally for additional strength.

Prostheses according to certain embodiments of the invention, which donot need to serve a posterior stabilization function, such as thosewhich can be characterized as cruciate retaining, need not have a postor other raised surface 422 on insert 400, or cams, such as cams 232 or230. In such embodiments and aspects, other surfaces such as portions ofthe medial and lateral condylar sections 202, 204 acting without a postor raised surface 422, for example, achieve or help achieve objectivesof aspects of the invention, including allowing or imparting internalrotation to the tibia relative to the femur as the knee flexes, such asfrom substantially 0 degrees to substantially 130 degrees.

Certain embodiments of the femoral component 200 may includeconventional attachment aids for helping to secure the femoral component200 to a distal end of a femur. Such attachment aids may include one ormore pegs, fins, surface treatments including bone ingrowth surfaces,surfaces for accommodating spacers, shims or other structures, or asotherwise desired.

Tibial components 300 according to certain embodiments and aspects ofthe invention include a tray 301 or base member for being secured to aproximal end of a tibia. The base member can include a stabilizing post,which is insertable into the tibial medullary canal and provides for thestabilization of the tibial component 300 on the tibia.

Tibial components according to embodiments and aspects of the inventionfeature a tray member which includes a proximal or upper surface, adistal or lower surface, a medial surface, a lateral surface, ananterior or front surface, and a posterior or rear surface. The proximalsurface may be substantially flat and planar. The tray member preferablyincludes attachment aids for helping to secure the tray member to aproximal end of a tibia. Such attachment aids may include one or morepegs, fins, screws, surface treatments, etc.

Femoral components 200 and tibial components 300 according to certainembodiments and aspects of the invention may be constructed in variousmanners and out of various materials. For example, the femoral component200 and tibial component 300 may be machined, cast, forged or otherwiseconstructed as a one-piece integral unit out of a medical grade,physiologically acceptable metal such as a cobalt chromium alloy or thelike, in various sizes to fit a range of typical patients, or may becustom-designed for a specific patient based on data provided by asurgeon after physical and radiography examination of the specificpatient.

Inserts 400 according to certain embodiments and aspects of theinvention include a proximal or upper surface 402, a distal or lowersurface 404, a medial surface 406, a lateral surface 408, an anterior orfront surface 410, and a posterior or rear surface 412. For convenience,such an insert 400 may be considered to feature a medial side 414 and alateral side 416, corresponding to medial and lateral sides of the limbin which the insert is to be installed.

The proximal surface 402 of the particular insert 400 according to oneembodiment of the invention shown in the drawings has a medial portion418 for engaging the outer surface 210 of the medial condylar section202 of the femoral component 200, and a lateral portion 420 for engagingthe outer surface 212 of the lateral condylar section 204 of the femoralcomponent 200.

Inserts 400 according to certain embodiments and aspects of theinvention can include a central post or raised portion 422 as shown inthe drawings. The post 422 includes a proximal surface 424, an anteriorsurface 426, a posterior surface 428 and medial and lateral sidesurfaces 430, 432. The anterior surface 426 of post 422 in an embodimentof the insert, is tapered or curved at a desired angle with respect tothe distal surface 404 of the insert 400 to minimize impingement of thepatella or a patellar implant 500 in deep flexion. The base can betapered as desired in a posterior direction from the anterior surface426 to minimize impingement of the intercondylar notch 208 of femoralcomponent 200 in hyperextension.

Inserts 400 of certain embodiments and aspects of the invention as shownin the drawings include an anterior curved surface. The anterior curvedsurface allows room for the patellar tendon (not shown). The insert mayalso include a posterior curved surface. The result of the posteriorcurved surface is the removal of material that may impinge on theposterior cortex of the femur in deep flexion. The radius of curvaturemay vary as desired to provide sufficient room for maximal flexion.

The distal surface of the insert 400 according to certain embodimentsand aspects of the invention may be substantially flat or planar forcontacting the proximal surface of the tray member 301 of the tibialcomponent 300. The distal surface preferably includes a dovetail orother appropriate locking mechanism that consists of an anterior portionand a posterior portion. However, any conventional method forpositioning and/or retaining the insert relative to the tray member 301,whether constrained or unconstrained, may be used. In other embodiments,the insert 400 may be allowed to articulate relative to the tray 301 ofthe tibial component 300.

On the proximal surface 402 of inserts 400 according to certainembodiments and aspects of the invention, parts of the medial portion418 of the proximal surface and parts of the lateral portion 420 areshaped to cooperate with outer surfaces 210 of the medial condylarsection of femoral component 200 and outer surfaces 212 of the lateralcondylar section of the femoral component, as the knee flexes andextends. These parts are referred to as medial insert bearing surface440 and lateral insert bearing surface 442.

From a sagittal aspect, as shown in FIGS. 4-11 and also in FIGS. 23 and24, posterior parts of the lateral bearing surface 442 of the particularinsert shown in the drawings features a reverse slope; that is, thelateral bearing surface slopes toward the bottom or distal surface ofthe insert 400 as the lateral bearing surface progresses toward theposterior or back periphery of the insert 400, preferably either througha convex arc or a straight slope. The purpose of the slope is to changethe direction of the contact force between the lateral bearing surface442 and the lateral condylar section 204, in order to add an anteriorforce on the lateral bearing surface 442 greater than a correspondinganterior force on the medial bearing surface 440 at some angles of kneeflexion, to produce or help produce a twisting moment about thelongitudinal axis of the tibia or impart or assist in imparting internalrotation of the tibia as the knee flexes. Preferably, thisrotation-imparting surface 444 is configured to impart or assist inwardtibial rotation relative to the femur as the knee flexes betweensubstantially 0 degrees of flexion to substantially 130 degrees offlexion, the internal rotation angle achieving a magnitude of at leastsubstantially 8 degrees at substantially 130 degrees of knee flexion.Since the contact force vector is perpendicular to the lateral bearingsurface 442, during rollback in the lateral compartment, a component ofthe contact force vector is generally parallel to the generallyanteriorly oriented contact vector acting on the post 422. Accordingly,this contact force not only can help delay engagement of the post 422with the posterior cam 232, but it can also beneficially reduce theforce required by the post 422 to produce lateral rollback, resistanterior motion of the femoral component 200 relative to the insert 400,and resist total force which is absorbed by the post 422 inaccomplishing posterior stabilization of the knee.

It is also possible to generate the tibial inward rotation inducingcouple on the insert 400 by the femoral component 200 not only by usingthe posterior cam 232 as discussed below, but also by altering the shapeof parts of the medial insert bearing surface 440 or using otherstructures, surface shaping or other techniques, or any combination ofthem, as desired.

Preferably, the lateral insert bearing surface 442 of the insert asshown in the drawings features a curved generally concave portion whichsweeps laterally from its anterior extremity to approximately itsmiddle, and then back medially from its middle to its posteriorextremity, as shown in FIG. 23, for example. Such a swept surface helpsguide the lateral condylar section 202 as the locus of its contactpoints with the insert 400 move in a posterior direction as the kneeflexes.

Inserts 400 according to certain embodiments and aspects of theinvention may be constructed in various manners and from variousmaterials. For example, they may be machined, molded or otherwiseconstructed as a one-piece, integral unit out of medical grade,physiologically acceptable plastic such as ultra-high molecular weightpolyethylene or the like, in various sizes to fit a range of typicalpatients, or may be custom-designed for a specific patient based on dataprovided by a surgeon after physical and radiographic examination of thespecific patient. The material can be treated, for example, byradiation, chemistry, or other technology to alter its wear propertiesand/or strength or hardness. Portions of various surfaces of inserts 400can be treated with radiation, chemicals or other substances ortechniques to enhance wear resistance properties; they can also besubjected to suitable surface treatments for such purposes and others.

If the medial condylar section 202 and the lateral condylar section 204of the femoral component 200 were the same size, the insert 400 shown inthe drawings would be thinner between its lateral insert bearing surface442 and its distal surface 404 than between its medial insert bearingsurface 440 and that distal surface 404. Such thinness may becomeunacceptable in regions between the rotation inducing surface 444 andthe distal surface 404 in the posteriolateral region of the insert 400.To compensate, lateral parts of the insert 400 may be created thickerthan medial parts, as shown for example in FIG. 20, so that the lateralinsert bearing surface 442 is “higher” or more proximal than the medialinsert bearing surface 440. In certain embodiments of the insert 400 asshown for example in FIG. 20, a line drawn between the most distal partof the medial insert bearing surface 440 and the most distal part of thelateral insert bearing surface 442 and denominated physiological jointline 227, forms an approximately 3 degree angle from a lineperpendicular to the mechanical axis of the leg or in many insert 400structures, substantially 3 degrees from the plane of the distal surfaceof the insert 400. This 3 degree angle is similar to the structure ofthe human knee, where the physiological joint line is usuallysubstantially 3 degrees from the mechanical axis of the joint. Thelateral contact point 436 of the femoral component 200 and the insert400 is initially higher than the medial contact point 434. Duringflexion, as the lateral condyle 204 rolls posteriorly, the lateralfemoral condyle 204 moves down the arc or slope of tibial rotationinducing surface 444 of insert 400.

In some cases, the epicondylar axis 242 (the line connecting the lateralepicondylar prominence and the medial sulcus of the medial epicondyle)could have a tendency to decline, which could cause rotation about thelong axis of the femur and might cause laxity of the LCL. According tocertain embodiments of the invention, it would be possible to keep theepicondylar axis 242 at the same height, by causing the sagittal curveof the posterior portion 224 of the lateral condyle 204 to be extendedoutwardly as could be visualized with reference to, for instance, FIGS.4-11. For example, at 155° flexion, the lateral contact point 434 coulddecline approximately 2.6 mm, so that 2.6 mm would be added to thelateral condyle 204 thickness at a point corresponding to 155° flexionon the condyle to accomplish such a result, although other structurescould be created to achieve the same end.

When assembled, the femoral component 200 shown in the drawings ispositioned on the insert 400 so that there is a slight posterioroverhang. This optimizes the anteriorposterior patella ligament forcecomponents. The overhang may be much less than in conventional kneeprostheses. For example, in conventional knee prostheses, the posterioroverhang of the femoral component 200 may be as much as 6 mm. However,in knee prosthesis according to certain embodiments and aspects of theinvention, the posterior overhang of the femoral component 200 isapproximately 2 mm.

As explained above, axial rotation is normal in knee joint motion. The“screwhome” mechanism is example of this motion. In the normal knee,during knee extension, the femur is positioned anteriorly on the tibialplateau. During the last 200 of knee extension, the femur glidesanteriorly on the tibia and produces external tibial rotation. Thisscrew-home mechanism in terminal extension results in tightening of bothcruciate ligaments and locks the knee such that the tibia is in theposition of maximal stability with respect to the femur.

When the normal knee begins to flex, posterior glide of the femur beginsfirst on the lateral tibial surface. Between approximately 0° and 130°of flexion, posterior glide on the lateral side produces relative tibialinternal rotation, a reversal of the screw-home mechanism.

Knee prostheses 100 according to certain embodiments of the inventionincorporate an allowance that mimics the screw-home mechanism. Thescrew-home allowance may be achieved by incorporating a swept surface onthe lateral surface 416 of the insert 400. The screw-home allowance isillustrated most clearly in FIG. 12. FIGS. 12-19 demonstrate that as theknee flexes from approximately zero degrees to approximately 130degrees, the femoral component 200 and the insert 400 rotate relative toeach other generally about a closely grouped set of medial contactpoints 436. As the knee flexes, the femoral component 200 rotatesexternally relative to the insert 400, which would be fixed on a tibialcomponent 300 in a fully assembled knee prosthesis 100; or consideredfrom the other perspective, the insert 400 and the tibia rotateinternally relative to the femoral component 200 and the femur. Theasymmetrical shape of the posterior cam 232 reduces force on the centralpost 422 that would oppose this rotation.

This rotation, along with the increased flexion of the knee prostheses100 of the invention, is evident in the series of side views of portionsof a knee prosthesis 100 shown in FIGS. 4-11. To demonstrate therotation between the femoral component 200 and the insert 400, whichwould be fixed on a tibial component 300 in a fully assembled kneeprosthesis 100, the insert 400 shown remains stationary, as the femoralcomponent 200 rotates substantially about the medial contact point.Thus, as shown in FIG. 4, the knee is fully extended. As the knee flexesto 90 degrees (shown in FIG. 7), the lateral condylar section 204 of thefemoral component 200 rotates posteriorly on the lateral side 416 of theinsert 400. The rotation continues as the knee flexes to 130 degrees, asshown in FIG. 9, reaching at least approximately 8 degrees of internalrotation of the tibia relative to the femur. As the knee continues toflex beyond approximately 130 degrees, as shown in FIGS. 10-11, theinternal rotation stays substantially the same, as the relative motionis primarily posterior translation of the femoral component on theinsert.

As the drawings show, when the knee prosthesis 100 is assembled, thecentral post or raised portion of the insert 400 fits within theintercondylar recess. Because the femoral component 200 and the insert400 are not fastened to each other, the femoral component 200 is able toeasily articulate on the insert 400.

FIGS. 4-11 thus sequentially show, from a side cross sectional aspect,kinematics of components of a knee prosthesis according to a preferredembodiment of the invention. FIGS. 12-19 show the same kinematics from aplan aspect, looking “down” on the prosthesis. These figures showkinematics of the prosthesis components at flexion angles of 0, 30, 60,90, 120, 130, 140, and 150 degrees, respectively. At flexion angles ofapproximately 50 to 60 degrees, the cam 232 begins contacting the post422 for posterior stabilization, as shown in FIG. 6. As the rotation ofthe femoral component 200 continues, the patella implant 500 moves downthe trochlear groove 206, which is structured according to aspects ofthe invention to simulate natural anatomy in order to allow the patellaimplant 500 to track properly, and generally from a lateral to medialposition relative to the femoral component 200 as flexion continues. Inthis fashion, the shape of the femoral component accommodates thenatural action of the kneecap as a fulcrum on the knee joint for theconsiderable forces applied by the quadriceps and the patellar ligament.As the knee flexes from substantially zero degrees of flexion tosubstantially 130 degrees of flexion, the tibial rotation inducingsurface 444 of the particular (nonlimiting) structure shown in thedrawings acting in combination with the lateral condylar section 204,plus the action of the asymmetrical posterior cam 232 of the femoralcomponent 200 on the post 422 of the insert, impart inward rotation tothe insert 400 relative to the femur. This inward rotation correspondsto such inward rotation in the normal knee, and allows, among otherthings, the lower leg to be “folded” inward relative to the upper leg sothat the patellar ligament and tendons from the quadriceps are notforced to be extended over the lateral part of the knee as is the casein some conventional designs. Yet the structure of the components shownin these drawings allows such natural internal rotation and othernatural articulation of the tibia and femur relative to each otherwithout freeing rotation of the insert relative to the tibial implant,or freeing other components in the prosthesis to move relative to eachother, thereby taxing the other, weaker ligaments and tendons formingpart of the knee, which are required to assume the new task ofrestraining the freed prosthetic components.

Designs more closely approximating the structure and/or operation of thenatural knee may be carried out according to the present invention byconsidering forces acting on the knee that are of more considerablemagnitude than other forces. For instance, 6 major forces on the tibiacan be used to simulate what a natural knee experiences during certainactivities such as walking. (1) ground reaction force which can rangefrom some part up to multiples of body weight in a normal knee kineticenvironment; (2) tension imposed by the quadriceps acting through thepatella tendon in a generally proximal direction tending toproximal-posterior in flexion and to proximal-anterior in extension; (3)tension applied by the hamstrings in a generally posterior direction;(4, 5) contact force of each condyle on its corresponding bearingsurface of the tibial plateau; and (6) posterior stabilization forceimposed by the posterior cruciate ligament or insert on the femur. Theinventors have recognized that reducing the myriad of forces acting onthe knee (such as from various more minor tendons and ligaments) to amanageable number, which may increase as time and processing powercontinue to evolve, allows for reliable and effective testing ofproposed knee prosthesis designs, by accurately simulating what realknees experience. This manageable set of conditions may be combined withinformation that is known about the structure and the kinematics ofnatural knees to impose an essentially realistic test regime forcomputer testing and development of acceptable knee prosthetic designs.

Applying a testing regime using a manageable but essentially realisticset of conditions allows iterative proposal of a design, testing it forperformance in virtual, automated fashion in a computer, modification ofthe proposed design to reduce negative performance characteristics andto enhance positive ones, and repeated iteration of these tasks until anacceptable design is reached. The developers may therefore accordinglyproceed at least partially iteratively, using test conditions thatsimulate what a real knee joint experiences and how it performs in suchan environment, rather than attempting to design the complicated kneeprosthetic components in a deterministic fashion based on anecdotalinformation, observation of knee components being articulated in theoperating room, or based on assumptions that can be static and notreflect the complexity of nature.

The foregoing is provided for disclosure of various embodiments, aspectsand structures relating to the invention. Various modifications,additions and deletions may be made to these embodiments and/orstructures without departing from the scope and spirit of the invention.

What is claimed is:
 1. A prosthesis for replacement of a least part of aknee joint in a leg, the leg including a femur and a tibia, theprosthesis comprising: a femoral component adapted to fit on a distalend of the femur, the femoral component comprising: a lateral condylarstructure defining a lateral condylar bearing surface; and a medialcondylar structure defining a medial condylar bearing surface; a tibialcomponent adapted to fit on a proximal end of the tibia, the tibialcomponent comprising: a medial bearing surface and a lateral bearingsurface, wherein: the medial bearing surface comprises a posterior-mostedge; the lateral bearing surface comprises a posterior-most edge; andthe medial bearing surface has a minimum thickness that is thicker in asuperior-inferior direction at the medial bearing surface posterior-mostedge than a minimum thickness of the lateral bearing surface at thelateral bearing surface posterior-most edge.
 2. The prosthesis of claim1, further comprising a central post positioned between the medialbearing surface and the lateral bearing surface.
 3. The prosthesis ofclaim 1, wherein at least one of the medial bearing surface or thelateral bearing surface is concave.
 4. The prosthesis of claim 1,wherein the posterior-most edge of the medial bearing surface is anoutermost edge of the medial bearing surface in a posterior directionand the posterior-most edge of the lateral bearing surface is anoutermost edge of the lateral bearing surface in the posteriordirection.
 5. The prosthesis of claim 1, wherein the medial bearingsurface and the lateral bearing surface each are concave.
 6. Aprosthesis for replacement of a least part of a knee joint in a leg, theleg including a femur and a tibia, the prosthesis comprising: a femoralcomponent adapted to fit on a distal end of the femur, the femoralcomponent comprising: a lateral condylar structure defining a lateralcondylar bearing surface; and a medial condylar structure defining amedial condylar bearing surface; a tibial component adapted to fit on aproximal end of the tibia, the tibial component comprising: a medialbearing surface and a lateral bearing surface, the medial bearingsurface comprising a posterior-most edge and the lateral bearing surfacecomprising a posterior-most edge, wherein the medial bearing surface hasan average thickness in a superior-inferior direction along the medialbearing surface posterior-most edge that is thicker than an averagethickness of the lateral bearing surface along the lateral bearingsurface posterior-most edge.
 7. The prosthesis of claim 6, furthercomprising a central post positioned between the medial bearing surfaceand the lateral bearing surface.
 8. The prosthesis of claim 6, whereinat least one of the medial bearing surface or the lateral bearingsurface is concave.
 9. The prosthesis of claim 6, wherein theposterior-most edge of the medial bearing surface is an outermost edgeof the medial bearing surface in a posterior direction and theposterior-most edge of the lateral bearing surface is an outermost edgeof the lateral bearing surface in the posterior direction.
 10. Theprosthesis of claim 6, wherein the medial bearing surface and thelateral bearing surface each are concave.
 11. A prosthesis forreplacement of a least part of a knee joint in a leg, the leg includinga femur and a tibia, the prosthesis comprising: a femoral componentadapted to fit on a distal end of the femur, the femoral componentcomprising: a lateral condylar structure defining a lateral condylarbearing surface; and a medial condylar structure defining a medialcondylar bearing surface; a tibial component adapted to fit on aproximal end of the tibia, the tibial component comprising: a superiorsurface configured to cooperate with the lateral condylar bearingsurface and the medial condylar bearing surface of the femoralcomponent, the superior surface comprising a medial bearing surface anda lateral bearing surface; and an inferior surface disposed on anopposite side of the tibial component from the superior surface,wherein: the medial bearing surface comprises a posterior-most edge; thelateral bearing surface comprises a posterior-most edge; and a distancebetween the posterior-most edge of the medial bearing surface and theinferior surface is greater in a superior-inferior direction than adistance between the posterior-most edge of the lateral bearing surfaceand the inferior surface.
 12. The prosthesis of claim 11, wherein: thedistance between the posterior-most edge of the medial bearing surfaceand the inferior surface is an average distance between theposterior-most edge of the medial bearing surface and the inferiorsurface along the posterior-most edge of the medial bearing surface; andthe distance between the posterior-most edge of the lateral bearingsurface and the inferior surface is an average distance between theposterior-most edge of the lateral bearing surface and the inferiorsurface along the posterior-most edge of the lateral bearing surface.13. The prosthesis of claim 11, wherein: the distance between theposterior-most edge of the medial bearing surface and the inferiorsurface is a minimum distance between the posterior-most edge of themedial bearing surface and the inferior surface along the posterior-mostedge of the medial bearing surface; and the distance between theposterior-most edge of the lateral bearing surface and the inferiorsurface is a minimum distance between the posterior-most edge of thelateral bearing surface and the inferior surface along theposterior-most edge of the lateral bearing surface.
 14. The prosthesisof claim 11, further comprising a central post positioned between themedial bearing surface and the lateral bearing surface.
 15. Theprosthesis of claim 11, wherein at least one of the medial bearingsurface or the lateral bearing surface comprises a concave portion. 16.The prosthesis of claim 11, wherein the posterior-most edge of themedial bearing surface is an outermost edge of the medial bearingsurface in a posterior direction and the posterior-most edge of thelateral bearing surface is an outermost edge of the lateral bearingsurface in the posterior direction.
 17. The prosthesis of claim 11,wherein the medial bearing surface and the lateral bearing surface eachcomprise a concave portion.
 18. A prosthesis for replacement of a leastpart of a knee joint in a leg, the leg including a femur and a tibia,the prosthesis comprising: a femoral component adapted to fit on adistal end of the femur, the femoral component comprising: a lateralcondylar structure defining a lateral condylar bearing surface; and amedial condylar structure defining a medial condylar bearing surface; atibial component adapted to fit on a proximal end of the tibia, thetibial component comprising: a medial bearing surface and a lateralbearing surface, the medial bearing surface comprising a medialposterior-most edge and the lateral bearing surface comprising a lateralposterior-most edge, wherein the medial bearing surface is thicker in asuperior-inferior direction at the medial posterior-most edge than thelateral bearing surface at the lateral posterior-most edge.
 19. Theprosthesis of claim 18, wherein the medial bearing surface is thicker inthe superior-inferior direction at an inferior-most point of the medialposterior-most edge than the lateral bearing surface at an inferior-mostpoint of the lateral posterior-most edge.
 20. The prosthesis of claim18, wherein each of the medial bearing surface and the lateral bearingsurface are concave.